Abdo, Yasmeen
(2019)
Structures, Energetics, and Vibrational Frequencies of Microhydrated Hexafluorophosphate, PF6−(H2O)n=1,2 from DFT and Ab Initio Computations.
Undergraduate thesis,
under the direction of Gregory Tschumper from
Chemistry and Biochemistry,
University of Mississippi.

Abstract

This study systematically examines an anion commonly used in room temperature ionic liquids, hexafluorophosphate PF6−, and its non-covalent interactions with up to two explicit water molecules (PF6 − (H2 O)n where n = 1, 2). Initial low-energy configurations are identified via a set of relaxed angular scans across the edges and faces of the PF6− octahedron using the global hybrid M06-2X density functional with a triple-ζ correlation consistent basis set augmented with diffuse functions on all non- hydrogen atoms (cc-pVTZ for H and aug-cc-pVTZ for P, O, F; denoted haTZ). Full geometry optimizations are performed on these initial structures using a variety of common density functionals theory (DFT) methods (B3LYP, B3LYP-D3, M06-2X, and ωB97XD) as well as the MP2 and CCSD(T) ab initio methods with the same haTZ basis set. The corresponding harmonic vibrational frequencies are computed for all identified stationary points. Single point energy computations are also performed on the CCSD(T)/haTZ geometries using the CCSD(T) method with an analogous quadruple-ζ basis set (haQZ). A new PF6−(H2O)2 minimum has been identified that is approximately 2 kcal mol−1 lower in energy than any other structure previously reported in the literature. For the PF6−(H2O)1 system, DFT computations identify two unique stationary points competing for the lowest energy configuration, which is consistent with prior work. However, only one of these structures is a stationary point on the MP2 and CCSD(T) potential energy surfaces. This result suggests that some DFT methods might not correctly describe the interaction between PF6−and H2O.